1. Field of the Invention
The invention relates to preparation of a free cured vulcanizate comprising a modified elastomeric composition. More specifically, the invention relates to a vulcanizate prepared from a rubber-resin blend wherein the resin particles which are dispersed in the rubber have a certain shape with the result that the necessity for restraining pressure to prevent blowing during the cure cycle of an article shaped from said blend is eliminated.
2. Description of the Prior Art
Presently, the blowing of rubber articles during or after cure (the terms "cure", "vulcanize" and variations thereof are used herein interchangeably) is prevented by the application of restraining pressure on the article being cured through the use of platens, mold forms or other suitable restraining means. The blowing phenomenon occurs because one or more of the organic or inorganic compounds which are purposely or inadvertently compounded into rubber vaporize or expand when heated during the curing step of the fabrication process. As a result the structure of the rubber is altered from a continuous solid to a cellular or spongy state.
Temperature and pressure are two important parameters when considering the vulcanization of any given rubber product. Because it is economically advantageous to have the fastest possible cures of rubber products when the product is held under restraining pressure, the temperature chosen is usually the highest possible temperature consistent with the production of goods of satisfactory quality.
However, when rubber products are cured without pressure, or at a relatively low pressure, it has been determined that the maximum temperature which can be employed in the curing step is limited by the tendency of most rubbers to blow during the cure, resulting in an undesirable porous article. The need for restraining pressure on the rubber product during the curing cycle frequently necessitates the addition of a separate step to the process as well as adding substantial expense to the overall cost of the product since curing equipment capable of exerting restraining pressure must be provided.
Among others, two ways in which the prior art sought to eliminate the blowing during cure are: (1) to remove the substances which would vaporize (i.e. the volatiles) in the rubber, or (2) to use rubbers possessing high green strength.
To remove the volatiles contained in a compounded rubber, the rubber product is heated to a relatively low temperature in a dry vacuum prior to the curing step. This method also necessitates additional costly equipment on the production line and is time-consuming.
The use of rubbers possessing high green strength eliminates the blowing phenomenon during the curing cycle because the uncured rubber composition has the inherent strength to withstand the bubble formation and expansion which generally results from the formation of entrapped gases, thus keeping them in solution in the rubber. The major drawbacks in this approach are that only a limited number of rubber polymers possess the necessary internal strength to withstand blowing, and further, the modulus of these rubber polymers is such that their commercial uses are somewhat limited.